1. Field of the Invention
The present invention relates to time delay and integrate (TDI) charge coupled device (CCD) sensors. In particular, the invention relates to discontinuous diagonal heat conducting straps on TDI CCD sensors to remove heat, a tab being included at each discontinuity to compensate for the extra illumination through the gap of the discontinuity.
2. Description of Related Art
Charge coupled devices (hereinafter CCDs) are widely used in video imaging and inspection applications. For example, a CCD sensor may be applied to industrial inspection or vision equipment. Inspection cameras used to inspect moving objects (e.g., on a continuous conveyor belt or rolled goods such as rolls of cloth) employ a line scan CCD sensor where a linear CCD sensor is oriented in a direction perpendicular to the direction of movement of the object being imaged. Advanced linear CCD sensors often employ a time delay and integrate technology and are referred to as TDI CCD sensors.
In FIG. 8, known time delay and integrate (TDI) linear array sensor 2 includes imaging section 4 coupled to horizontal CCD readout shift register 6. Imaging section 4 includes a plurality of columns, each column including a plurality of photo-sites. In operation, a camera lens focuses the image on the TDI CCD sensor. The image, that is the optical input to the TDI CCD sensor, is moving (e.g., on a conveyor belt). An image conjugate focused on the sensor appears to be moving. A portion of the image first appears on one pixel of the TDI CCD sensor, and then appears on another pixel of the sensor. The camera and sensor are arranged so that a portion of the moving image moves in a direction from the top of column 8 of photo-sites to the bottom of column 8 (adjacent to horizontal readout shift register 6). The TDI CCD sensor is clocked to transfer charge down the columns of photo-sites at a rate equal to the rate that the portion of the image moves down the column. Charge generated at a first photo-site is transferred to the next photo-site at the same time that the image portion that generated the charge at the first photo-site moves to the next photo-site. In this way photo-charge is accumulated at the photo-site under the image portion as the image portion moves down the column. Thus the name, time delay and integrate (TDI).
In a TDI CCD sensor, the last pixel in each integrating column (e.g., column 8) of photo-sites (i.e., the last horizontal line) is transferred into a horizontal CCD readout shift register. The line of data is then transferred serially through an output node structure at the end of the register and then through a buffer amplifier.
Imaging section 4 includes a plurality of such integrating columns. Disposed over, and insulated from, the columns is a plurality of sets of clock driving lines, one clock driving line in each set is for a corresponding phase of the clock system. To transfer signal charges vertically in each column, clocking signals are applied to corresponding clock driving lines in each set of clock driving lines. The clock driving lines are typically formed of doped poly-crystalline silicon and are characterized by a high resistance "per square" when compared to metal. Thus, metal strapping is sometimes employed over the poly-crystalline silicon lines to provide a low resistance shunt for the clock signals to provide a more even distribution of clock signals over the entire area of imaging section 4.
U.S. Pat. No. 4,375,652 to White describes a time delay and integrate scanner with opaque metal conductors extending diagonally across the light receiving surface of the array to apply control signals.
However, in high density, large format CCD sensors, significant heat can build up in the center of the array unless a means is provided to remove the heat. Known heat removal means rely on conduction through the substrate of the CCD sensor. Heat that is not removed causes dark signal noise to be inserted in "hot" pixels. In fact, warmer pixels will experience more dark signal noise than will cooler pixels. To produce a quality image, it is necessary to provide uniform thermal as well as electrical and optical performance across the entire imaging area of the sensor.